AP STATISTICS4.2D Blocking and Matched Pairs Design

Housekeeping

Unit 1 Test Thursday and Friday (9/24 & 9/25) Multiple Choice Thursday FRQ Friday

These are timed tests—you must finish each section within the time allotted

Homework packets due ON ENTRY, Friday, 9/25

Team Time

Go over 4.2D HW

Opener: Doing Laundry

Researchers want to test whether a new detergent for clothes that require hand-washing cleans better in warm or cold water. They decide to perform an experiment using numerous pieces of dirty laundry as the experimental units. The response variable is a cleanliness rating on a scale of 0 (very dirty) to 10 (very clean).

Design a completely randomized experiment.

Opener: Doing Laundry

How should researchers deal with the fact that light-colored clothes come cleaner in cold water?

How could they ensure that after being assigned to a block, the clothes are randomly assigned a treatment?

The random assignment of experimental units to treatments is carried out separately within each block.

Many piles of dirty laundry

Assignment to blocks is NOT random

Dark-colored clothing

Light-colored clothing

Random assignment

Random assignment

Hot water

Cold water

Hot water

Compare cleanliness

Compare cleanliness

Combine results

Cold water

Why do we block?

How does blocking bring control into the laundry experiment?

Control what you can, block what you can’t control, and randomize to create comparable groups.

Matched Pairs

Common type of randomized block design. Create blocks by matching similar pairs of

experimental units (blocks of size 2). Use chance to decide which member of a pair gets

the first treatment (random assignment of treatments within each pair).

Helps reduce the effect of variation among experimental units.

Sometimes each “pair” is one experimental unit who receives two treatments. The order of the treatments is randomized for each experimental unit.

Stand Up, Sit Down! LAB

Are standing pulse rates higher than sitting pulse rates? If you received a black card, you will begin by sitting down.

If you received a red card, you will begin by standing up. When I tell you to begin, keep track of your heart rate. It

will take one minute, so please focus. Record this number. If you were standing, please sit down. If you were seated,

please stand. Again, we will record the number of heart beats in one

minute. Record your standing and sitting heart rates on your card. Turn in your card

Homework

4.2D HW: Blocking and Matched Pairs Design